Interactions between the fibers and matrix of continuous fiber reinforced composite materials that occur during manufacture and in use determine the mechanical properties, fracture behavior, and service characteristics of these materials. The inherent fiber-matrix compatibility and the composite manufacturing process must result in a strong bond between the fibers and the matrix and, at the same time, minimize the dissolution of the fiber in the matrix and reaction diffusion between the matrix and the fiber materials. The lack of a strong bond between the fibers and the matrix, the dissolution of the fibers in the matrix, and reaction diffusion at the fiber-matrix interface cause bond breaking, delamination, internal stress, fracture, and other types of failures.
Conventional solutions to these problems include 1) the surface treatment of fibers with barrier coatings to preserve the fiber and the interface boundary strength, 2) the development of various combinations of compatible matrix and fiber materials, and 3) the addition of particulate additives to more evenly distribute fibers in the matrix throughout the composite and to serve as an interlocking mechanism between the fibers and matrix.
In the area of surface treated fibers, U.S. Pat. No. 4,097,624, June 27, 1978 to Schladitz describes a method of coating glass or carbon fibers with 3 .mu.m-thick Ni layer to minimize the fiber-matrix interaction. Similarly, the coating of carbon fibers with Mo or Cr layers followed by application of a SiC barrier coating is described in "Composite Materials," Baikov Institute for Metallurgy, USSR Academy of Sciences, Moscow, Nauka Publishing House, 1981, p. 71 and in D. Clark, N. J. Wadsworth, and W. Watt; "The Surface Treatment of Carbon Fibers for Increasing the Interlaminar Shear Strength of CFRP" in Carbon Fibers; Place Mod. Techno. London, 1974, p. 44-51.
Unfortunately barrier coatings using prior art methods and materials do not result in the simultaneous achievement of good adhesion between the fiber and the matrix and the prevention of interaction-diffusion at the interface. Carbide, nitride, or oxide barrier layers with high-temperature chemical stability hinder the formation of strong physico-chemical bonds between the matrix and the fiber resulting in separation of fibers from the matrix and the conglomeration of individual fibers. As a result, cracks tend to develop at the point of fiber to fiber contact.
A second method to achieve stronger composite materials is by enhancing the fiber-matrix bond through the improvement of the wettability of the fibers. This is achieved by coating the fibers or doping the matrix material. For example, carbon fibers are coated with less than a 1 .mu.m thick nickel layer to improve their wettability by an aluminum melt; aluminum is doped with up to 7% silicon and 0.6% magnesium to alter the adhesion mechanism at the fiber-matrix interface. "Cast Reinforced Metal Composites," Conference Proceedings, ASM International, 1988 p. 67. Although this approach enhances the physicochemical bonding between the fibers and matrix, it, unfortunately, causes dissolution of the fibers and the formation of intermediate phases and compounds that lessen the mechanical strength of the composite.
A third approach to obtaining stronger composite materials has been to use particulates that separate the fibers and interlock fibers and matrix. S. Towata and S. Yamada, "Composites 86: Recent Advances in Japan and the Untied States," Ed. Proc. Japan-U.S. CCM-III Tokyo, 1986, pp. 497-503, describe an aluminum alloy reinforced with continuous carbon or silicon carbide fibers in which silicon carbide whiskers or other fine particles are distributed among the continuous fibers. Soviet researchers have grown SiC whiskers directly on the bare surface of carbon fibers and then have incorporated the fibers into polymer-matrix composites. "Composite Materials based on Whiskered Fibers" and published in "Fiber-like Crystals and Thin Films", Proceedings of the IInd All-Union
Scientific Conference, Voronezh 1975, p. 373. In both of these cases, the shear strength of the composite material increased; however, because there is an absence of a barrier coating on the continuous fibers, disintegration of the fibers caused either by a chemical reaction between the fibers and the matrix material or by the whiskering process or both takes place resulting in a significant loss of composite material strength. When carbon fibers were whiskered at 3 wt. % to obtain optimal composite shear strength, the carbon-fiber strength decreased by a factor of two.